The invention relates to optical pellicles. More specifically the invention relates to optical pellicles that include an electrically conductive coating.
Integrated circuits are produced through the process of photolithography. Photolithography frequently employs ultraviolet light (UV) to reproduce a photomask or reticle pattern on a substrate, such as a silicon wafer. The photomask blocks transmission of a patterned portion of the incident light, allowing the inverse pattern to be focused on a photosensitive surface of the substrate. This optical process, followed by development of a positive or negative pattern on the substrate, facilitates creation of an integrated circuit. Superposition of related patterns through repetition of this process with multiple photomasks results in a multi-layered integrated circuit on the substrate.
Accurate reproduction of the photomask pattern on the substrate is critical to production of a functional microcircuit. Therefore, the integrity of the photomask must be protected to allow repeated use. Small particles, such as airborne dust or fibers, are a significant source for degrading the accuracy of photomask pattern reproduction. Even very small particles can alter light transmission when positioned near the focal plane of the photomask. As a result, these particles can produce defects in the microcircuit.
To protect the integrity of the photomask pattern, an optical pellicle is used. The optical pellicle includes a thin, uniform membrane. Typically, the optical pellicle is supported above the photomask surface by a frame. The membrane acts as a dust cover that is capable of keeping particles away from the surface of the photomask. Instead, particles are collected on the pellicle surface, at a distance from the photomask generally determined by the height of the frame. These particles are positioned relatively distant from the photomask focal plane, so that the ability of the particles to block light transmission to the photomask is significantly mitigated.
An effective optical pellicle is capable of very efficient transmission of incident radiation, with little distortion. To achieve these optical properties, pellicles are generally constructed of a material that absorbs very little light at the wavelength of light selected for the photolithographic process. This high transmissivity is coupled with a uniform thickness, in the range of approximately 0.5 xcexcm to 2 xcexcm. When sources that produce UV light of longer wavelengths are used in photolithography, nitrocellulose or cellulose acetate provides pellicle membranes with high transmissivity, but also require an anti-reflective coating due to the relatively high refractive index of these materials.
The wavelength of electromagnetic radiation used in the photolithographic process is directly related to the minimum feature size of the circuit produced on the substrate. Therefore, efforts to increase the density of the circuitry on microchips has caused photolithography to evolve from the use of mercury lamp g-line and i-line output at 436 nm and 365 nm towards deep-UV and vacuum-UV regions of the ultraviolet spectrum. The predominant spectral output of a mercury lamp occurs in the mid- to near-UV region. Thus the fabrication industry has turned to excimer lasers to produce microcircuits using radiation from the deep-UV region. For example, Krypton Fluoride (KrF) lasers produce UV radiation of 248 nm, whereas Argon Fluoride (ArF) lasers emit at 193 nm. For purposes of the present invention, deep-UV is defined as ultraviolet light with a wavelength less than 250 nm.
Fabrication of microcircuits through photolithography in the deep-UV region requires an optical pellicle with low absorption of this ultraviolet radiation. Fluoropolymers have been found to have the desired properties at 248 nm and 193 nm. Specifically, most effective polymers are amorphous, fluorine-containing polymers, and more specifically perfluoro amorphous resins, such as those described in U.S. Pat. No. 5,674,624 issued to Miyazaki et al, which is hereby incorporated by reference. Membranes constructed from commercially available fluoropolymer resins have been used successfully. For example, the fluoropolymers CYTOP from Asahi Glass and AF-1600 from DuPont have been found to be suitable. Pellicles constructed with these fluoropolymers show high transmissivity in the deep-UV range, have a low enough refractive index that an anti-reflective coating is generally not required, and are sufficiently resistant to damage by ultraviolet radiation.
Although fluoropolymer membranes show desirable optical properties for photolithography, they have undesirable electrical properties. Specifically, fluoropolymers act as extremely effective insulators. In fact, fluoropolymers suitable for use in an optical pellicle have very low dielectric constants, usually a value of less than 2.0, and a volume resistivity of greater than 1018 xcexa9-cm. The extremely effective insulating property of fluoropolymers prevents a standard fluoropolymer membrane from readily dispersing accumulated electrostatic charge. The result is a build-up of electrostatic charge on the fluoropolymer membrane, producing periodic, significant electrostatic discharge to the photomask. This electrostatic discharge is capable of degrading features of the photomask, especially with repeated electrostatic discharge. The problem of electrostatic discharge is further exacerbated by the small feature size of photomasks used in the deep-UV range. The mass of these features is frequently not sufficient to dissipate heat produced by electrostatic discharge, thus resulting in electrostatic discharge-mediated damage to the photomask. The ability of electrostatic discharge to damage photomasks results in the destruction of many thousands of photomasks every year, a substantial monetary loss to the microcircuit fabrication industry.
Efforts to minimize the effects of electrostatic discharge have focused on dissipating electrostatic charge in a clean room before and during microcircuit fabrication. For example, tools and instruments have been grounded and ionized air has been introduced into the clean room. This has lessened the impact of electrostatic charge accumulation during microcircuit fabrication but does not eliminate the effect of electrostatic discharge from outside sources.
One such outside source of electrostatic discharge is the optical pellicle itself. Because it is such an excellent insulator, the pellicle membrane may accumulate a charge when it is manufactured. Additional charge may also build up on the optical pellicle during shipping to its site of use.
Efforts to reduce charge build-up by coating fluoropolymer optical pellicles with an antistatic material have been unsuccessful, at least in part because electrically conductive materials are difficult to adhere to a surface of the fluoropolymer membrane. Therefore, the fabrication industry requires a charge dispersing material that will adhere to a deep-UV pellicle membrane to provide an antistatic pellicle.
The present invention provides an optical pellicle with an antistatic coating, and a sol-gel process for producing the coating. The coating comprises a thin, metal oxide layer that maintains high transmissivity of the optical pellicle to deep-ultraviolet light. In the sol-gel process disclosed, the thin metal oxide layer is produced by applying a sol to the pellicle membrane so that the sol is converted to a gel, and then drying the gel to a coating.